Method and apparatus for drying the fuel of a fluidizied-bed boiler

Abstract

The present publication discloses a method and an apparatus for drying the fuel used for firing a fluidized-bed boiler (1). The hot, inert solids of the fluidized bed are circulated by a regulated rate from the furnace (2) to a simple dryer (11) incorporated into the fuel feed line (7) to the boiler (1), whereby the fuel is dried and steam is generated. By virtue of controlled bed solids recirculation, a constant temperature of the dryer (11) can be maintained, thus permitting the omission of all heat transfer surfaces from the dryer (11). The nearly clean steam released from the drying process is routed from the dryer (11) to useful applications. (FIG. 1)

I claim: 
     
       1. A method for improving the energy economy in drying the fuel used for firing a fluidized-bed boiler (1), comprising the steps of: recirculating hot bed solids from a boiler furnace of the fluidized-bed boiler (1) to a dryer (11) incorporated into a fuel feed line,   mixing the fuel in the dryer (11) with the bed solids and drying the fuel, whereby steam is released from the fuel,   feeding the mix of the dried fuel and bed solids into the fluidized-bed boiler (1),   controlling the recirculation rate of the bed solids such that the temperature of the bed solids/fuel mix in the dryer (11) is kept above the saturation temperature of the steam, yet below the pyrolysis temperature of the fuel, and   routing the nearly clean steam generated in the drying process from the dryer (11) to useful applications.   
     
     
       2. A method as defined in claim 1, wherein the drying process is under pressure. 
     
     
       3. A method as defined in claim 1, wherein the recirculation rate control is implemented by regulating the amount of recirculated bed solids entering the dryer. 
     
     
       4. A method as defined in claim 3, wherein the recirculation rate control is implemented by means of a control element (15) and/or (16). 
     
     
       5. A method as defined in claim 1, wherein the operating temperature of the dryer (11) is maintained 0°-50° C. above the saturation temperature of the steam generated in the drying process. 
     
     
       6. A method as defined in claim 1 wherein the steam generated in the drying process is condensed. 
     
     
       7. A method as defined in claim 1, wherein the steam generated in the drying process contains a maximum of 5% inert gases. 
     
     
       8. A method as defined in claim 1, wherein the fuel to be dried is a water-containing fuel such as peat, coal, brown coal, sewage sludge, or biomass. 
     
     
       9. An apparatus for drying fuel used for firing a fluidized-bed boiler (1), said apparatus comprising a fluidized-bed boiler (1),   a fuel dryer (11), which is a fluidized-bed dryer,   a nozzle (8) for recirculating bed solids from the boiler to the dryer,   a nozzle (10) for feeding the mix of the fuel with the bed solids from the dryer to the boiler,   control elements (16) and/or (15) for regulating the amount of recirculated bed solids entering the dryer,   a nozzle (9) for recirculating a portion of the steam generated in the drying process to the dryer (11) to act as a fluidizing gas, and   no separate heat transfer surfaces in the dryer (11).   
     
     
       10. An apparatus as defined in claim 9, wherein said dryer (11) is pressurized.